Die-casting process is widely used in automobiles, mechanical equipment, home appliances and 3C products. Commonly used die-casting materials include aluminum alloy, zinc alloy, magnesium alloy, copper alloy, etc., of which aluminum alloy accounts for 60% to 80%. Die casting has characteristics of high dimensional accuracy of castings, high production efficiency, and can produce products with complex structures. However, since it is formed under high temperature and high pressure, and material undergoes a transition from liquid to solid, volume will change during cooling process, so shrinkage cavities and defects are more likely to occur in casting. At present, there are many technological methods to solve problem of shrinkage cavities inside castings. Among them, local pressurization technology has been widely used in die-casting production, which can effectively avoid occurrence of shrinkage cavities in locally thick parts of castings. This study takes an aluminum alloy bracket die-casting as an example. By using local pressurization technology to solve local shrinkage cavities and shrinkage porosity defects in casting, structural characteristics and quality requirements of aluminum alloy bracket die-casting are introduced. Difficulty lies in internal quality control of casting at four mounting feet, which requires its internal quality to meet porosity D5 requirement. Local pressurization technology is used in design of die-casting mold. Process parameters of local pressurization include extrusion pressure, extrusion time, and extrusion stroke. Orthogonal test methods are used to optimize process, providing a reference for solving similar problems.
Graphical results
Figure 1 shows an engine bracket produced by an automobile factory. Material is AlSi9Cu3(Fe) alloy of European standard EN1706. It is a frame structure. General wall thickness of part is 4.5mm. There are bolt mounting protrusions of different structures on both sides, wall thickness is relatively thick. Main function is to connect and fix engine to body frame to play the role of "support" and "vibration isolation". It is a stressed part. Internal structure of aluminum alloy bracket die-casting is required to be dense, without casting defects such as shrinkage and porosity, porosity is required to reach D5 (VW50097 standard). Sample was polished with a 3 μm silica suspension, and its microstructure was observed using an AX10 Zeiss metallographic microscope. Porosity was detected and calculated through special software.

 

Figure 1 Engine bracket

 

Figure 2 Simulation diagram of water filling on one side
1. Large inner cavity core-pulling slider 2. Casting 3. Slider 4. Squeeze rod 5. Squeeze cylinder
For this engine bracket, location that is difficult to meet D5 requirements is bolt mounting boss with thick walls at both ends. Shrinkage holes and shrinkage porosity can be reduced by setting sprues, pre-cast holes, and adjusting die-casting parameters (casting pressure, speed) to meet quality requirements. Bolt mounting bosses at the four legs of casting are scattered at four lateral corners, making it impossible to set up sprues. At the same time, bolt mounting holes are at an angle of 15° with mold opening direction of lateral slider, which side core pulling of pre-cast hole cannot be achieved in mold structure. Therefore, a local pressurizing mechanism is used to solve shrinkage holes at bracket legs. Mold structure is shown in Figure 2, using 2 cavities. In order to ensure consistency of filling conditions of two cavities, it is also considered that each casting has two core-pulling mechanisms in 90° direction, so layout of two cavities is 90°. Local supercharging mechanism is installed on large slide block that pulls core out of inner cavity of casting. Extrusion cylinder is installed at rear end of slide block to drive extrusion rod to move inside slide block.
Local pressurization technology, also known as local extrusion, means that after molten metal fills mold cavity during die-casting process, cooling and solidification process, extrusion rod is driven through oil cylinder at thick wall to repressurize unsolidified molten metal to increase local density and improve or even eliminate shrinkage cavities in pressurized parts. Main process parameters involved in local pressurization technology are extrusion pressure, extrusion volume, and extrusion time.

Table 1 Orthogonal test factor table

Table 2 Orthogonal test results
Through analysis of extrusion process of engine bracket, it can be determined that key process parameters that affect internal quality of aluminum alloy engine bracket after partial pressurization are extrusion pressure, extrusion stroke and extrusion start time, which are used as influencing factors. Each factor takes 3 levels. Through calculations and empirical data during structural design of local supercharging mechanism, it is determined that extrusion pressure is 200~400MPa, extrusion stroke of extrusion rod is 6~10mm, and extrusion start time is 0.5~ 1.5s. Orthogonal test factors and levels are shown in Table 1. Orthogonal test uses detection value of porosity as evaluation index. In order to ensure stability of test data, each set of process parameters was continuously die-cast for 20 molds, 3 pieces from middle injections were taken for porosity testing, and average value was taken. Orthogonal test table and porosity test results of aluminum alloy engine bracket are shown in Table 2, and extreme value analysis is shown in Table 3. Extrusion start time has the greatest impact on porosity of casting, followed by extrusion stroke and extrusion pressure. In order to more intuitively reflect influence trend of different factors on porosity of castings, experimental factors are used as abscissa and average porosity is used as ordinate to obtain influence trend of porosity with changes of each test factor.

Table 3 Range value analysis table

 

Figure 3 Trend of influence of test factors on porosity value
In conclusion
(1) Aluminum alloy bracket die-casting parts have local thick-walled areas at mounting boss. Local pressurization technology can effectively reduce shrinkage cavities and improve internal quality of thick-walled castings.
(2) Key process parameters of local extrusion are extrusion pressure, extrusion start time and extrusion stroke. Orthogonal experiments are used to obtain optimized extrusion process parameters.
(3) For aluminum alloy engine bracket castings, the biggest factor affecting internal quality after extrusion is extrusion start time, followed by extrusion pressure and extrusion stroke. Optimized extrusion process parameters: extrusion pressure is 300MPa, extrusion start time is 1.0s, and extrusion stroke of extrusion rod is 8mm.